This invention relates to an improved method of making a quadrupolar focusing color-selection structure for a CRT (cathode-ray tube).
A shadow-mask-type color television picture tube, which is a CRT, comprises generally an evacuated envelope having therein a target comprising an array of phosphor elements of three different emission colors arranged in color groups in cyclic order, means for producing three convergent electron beams directed toward the target, and a color-selection structure including an apertured masking plate closely spaced from the target. The masking plate shadows the target, and the differences in convergence angles permit the transmitted portions of each beam to select and excite phosphor elements of the desired emission color.
One family of color-selection structures is referred to as electrostatic quadrupolar focus masks. These structures have, as a common feature, an array of quadrupolar electrostatic focusing lenses, each lens consisting of metal electrodes held in closely-spaced relation by electrical insulators, which provide structural integrity for the structure and hold off the voltage required for focusing.
One such structure consists of two spaced sets of spaced, parallel electrode strips at right angles to one another forming apertures therebetween. In a typical focus mask, the apertures are on about 0.77 mm (30 mil) centers in the horizontal direction and 0.46 to 0.77 mm (18 to 30 mil) centers vertically. The electrical transmission of such a structure is about 50%, and the voltage across the two sets of strips during operation is about 600 volts dc. A second such structure comprises an apertured plate onto which is bonded a set of parallel strip electrodes such that each aperture is bisected by an electrode strip. A third structure comprises an apertured plate onto which is bonded a set of parallel strip electrodes such that there are electrodes on opposite sides of each aperture of the plate. The dimensions of and voltages employed in the second and third structures are similar to those of the typical structure mentioned above.
In any of these and similar structures, it is a major problem to construct, in a single focus mask which may be about 38 by 51 centimeters in size, an array of about 250,000 or more lenses that are precisely sized and positioned as is required for use in a CRT. Though not obvious, new fabrication techniques and new materials have been devised for other unrelated purposes which can be adapted toward this end.